Current land, air or sea going vehicle position determination arrangements most typically relies on the Global Positioning Systems (GPS). The GPS system comprises the use of three or more space vehicles (eg. satellites) in Low Earth Orbit that emit signals which when simultaneously received by a suitable receiver mounted to a vehicle can assist in the determination of the position of the vehicle. Specialised algorithms including triangulation techniques are used using the known location of three or more space vehicles. While the number of space vehicles received simultaneously is preferably four. The likelihood that a vehicle will receive signals from four space vehicles at any one time will vary with the particular GPS system used. However, 18 space vehicles in a suitably high orbit at any one time can provide a high probability of reception and broad coverage over the surface of the earth.
What a GPS receiver can provide is four variables. The first three are the x, y and z of the spatial coordinates of the mobile in a predetermined frame of reference. The fourth is its temporal coordinate (ie. the date at which the mobile is at the calculated position, this date being specified in universal time).
Commercial low cost GPS receivers at this time offer a 95% probability of providing calculated horizontal position solutions with errors of 15 meters or less. In other words, there is a 95% probability that the error in the horizontal position is 15 meters or less. Such figures are frequently quoted by GPS receiver manufacturers.
This level of accuracy is indicative when the GPS receiver is operating under ideal conditions. However, when the GPS receiver is operating in non-ideal conditions, such as those found in many urban areas, the observed performance is significantly worse. It is common to observe position errors of hundreds of meters or for the 95% error probability to be in excess of 40 meters.
However, this accuracy and the availability of the position solution can be greatly improved when the GPS solution is combined with speed and heading sensors associated with the vehicle to “dead reckon” the position of the vehicle from a known starting location or the last calculated position.
In some of the more expensive GPS receivers the integration of the positioning data from the GPS receiver to the dead reckoning sensors in the vehicle is tightly bound to the particular characteristics of a specific manufacturers GPS receiver. This means that the dead reckoning sensors provide their data direct to the proprietary algorithms used by the GPS receiver to calculate its position and time. In some respects such an approach is preferable since each manufacturers GPS receiver will know best how to deal with the vagaries of the dead reckoning sensors (eg. their particular accuracy's and operation). The GPS receiver will also know how best to introduce the dead reckoning data into the specific algorithms developed for the GPS receiver. In this case the statistical certainty of the dead reckoning sensor measurements provided to the GPS receiver is dealt with only once by a Kalman filter designed specifically to process such figures.
The practical outcome however is that users are locked into a particular GPS/dead reckoning vendor, the alternative being a major re-engineering effort when the GPS element of the system needs to change or the dead reckoning inputs change or to find an totally new GPS solution.
Thus, there exists a need for an accurate, flexible and cost-effective dead reckoning technology that can interface with any GPS receiver. Furthermore, and in accord with features that are available in the most expensive GPS systems it is an aim of the invention described herein to provide a method and means for qualifying and quantifying the quality of horizontal position data emanating from the chosen GPS receiver. In a practical embodiment of the invention, it produces a Figure Of Merit (FOM). The FOM can then be used to provide the best possible statistical certainty of the solution for position provided by the chosen GPS receiver.
Preferably, in providing such an approach, it is useful to use the industry standard NMEA-0183 data output by all GPS receivers as the basis for this determination. However not all the possible messages are made available by all GPS receivers. Thus, it is necessary in the embodiments described herein to use the minimum number of the most commonly available NMEA-0183 messages. This approach will allow the use of most if not all GPS receivers and selected dead reckoning sensors and their outputs and even the GPS receiver itself as inputs (ideally those dead reckoning sensor outputs readily available in current vehicles) so as to increase the availability and timeliness of a position solution of any GPS receiver, in particular those that are at the cheap end of the price range.
The industry standard for the format of serial digital messages that are output by GPS receivers is defacto governed by a United States organisation, the National Marine Electronics Association (NMEA) in standard NMEA-0183. The NMEA 0183 Interface Standard defines electrical signal requirements, data transmission protocol, time, and specific sentence formats for a 4800-baud serial data bus.
Very few automotive grade GPS receivers provide a FOM output. Those few that do, provide it via the manufacturer's proprietary message outputs. This is quite understandable, as the NMEA-0183 interface does not provide a mechanism for providing this information. Previously, it has been usual to use the various dilutions of precision (Position Dilution of Precision (PDOP), Horizontal Dilution of Position (HDOP) etc. and other measures such as signal to noise ratios and numbers of space vehicles used in the solution to formulate a view on solution quality. Such methods however, are largely insensitive to distortions arising from urban canyon effects. The present invention introduces a method and apparatus for determining a FOM that responds well to such error sources.